What is one of the main functions of a safety device in a hoistway?

One of the main functions of a safety device in a hoistway is to ensure that the elevator does not descend too rapidly. Safety devices, such as overspeed governors, are crucial for monitoring the speed of the elevator car. If an elevator starts to descend faster than its normal operational speed, these devices activate to engage the brakes, thereby preventing a potentially dangerous situation that could lead to accidents or injuries. This mechanism is part of the overall safety system designed to protect passengers and maintain control over the elevator's movement.

In the context of the other options, while providing power and preventing unauthorized access are important for the overall functioning and security of the elevator system, they do not directly relate to the specific safety functions required to control the descent speed. Maintaining cabin temperature is also essential for passenger comfort but does not pertain to safety functions designed to prevent accidents during the elevator's operation.